The expression 35 - m is used to determine the number of gallons left in the tank after m minutes.
Solution:
Given that a tank containing 35 gallons of water is leaking at a rate of gallon per minute.
Let "m" represent the number of minutes spent
Number of gallons left in the tank = initial amount of water present – leaked amount of water
Number of gallons of water left = 35 gallons – number of minutes spent
rate of leakage.
Number of gallons of water left = 35 gallons – m minutes
1 gallons per minute

Hence, the required expression is 35 – m